Pattern recognition of the bits in a binary bitstream is almost as old as the perking of the attention of a 19th century telegrapher as he hears the call letters of his station. Before the electronic era, mechanical sequence detectors would recognize a binary bit sequence, code combination or character and would prime the detector to recognize the next character in the sequence. When the last character in the expected sequence was thus primed and recognized, an electrical contact was closed or a mechanical lever was actuated.
With the arrival of semiconductors and integrated circuits, shift registers were used to convert the serially-arriving bits into simultaneous or parallel presentations of each binary code combination or character (now often called a "byte" if consisting of eight bits). AND-gates, NOR-gates, registers, etc., were then used to recognize the characters and note the order of their arrival, issuing some kind of trigger when the expected sequence of characters had been recognized.
With modern, high-speed data communications and electronic digital computer switching of such data traffic, the expected sequences to be recognized get longer, must be changeable under software control rather than by hard wiring, and the recognition circuits must react more quickly.
U.S. Pat. No. 4,298,987 granted, on Nov. 3, 1981, to Stattel, et al. teaches using the received code combination as the address for a random access memory (RAM) integrated circuit, the code combination that addresses the appropriate address location within the RAM causes a recognition signal to be issued at the input/output port of the RAM. RAMs are quick-acting electronic devices and their contents can be changed under software control. However, as taught by Stattel, et al., they can get very large and complicated when the sequences get long.